When choosing hair dyes, it is natural for the user to assume that the final hair color will resemble the color indicated on the product container or color chart. Often, however, the result is quite different. The ability to accurately predict resulting hair color after dyeing is essential, but not easy to attain because hair varies significantly from person to person. In this paper, a new method for predicting the dyed hair state is proposed. Our method is based on a model, where the observed final color consists of contribution from three layers of the hair structure: a melanin-containing layer, a bleached layer and a pigment-colored layer. A method for predicting the final color from responses in each layer is introduced. In this method, response formulae for each layer are derived from the energy balance, and the predicted hair color after dyeing is obtained by calculating the each equation assuming that the hair state before dyeing is solely because of the contribution from the melanin-containing layer. In addition, a small measuring apparatus utilizing this prediction method was developed and the effectiveness of our method demonstrated by data obtained before and after the hair dyeing.